diff --git a/rp06.cpp b/rp06.cpp
index b9066fd..00761cc 100644
--- a/rp06.cpp
+++ b/rp06.cpp
@@ -1,5 +1,6 @@
 // (C) 2024 by Folkert van Heusden
 // Released under MIT license
+// Some of the code is translated from Neil Webber's PDP11/70 emulator
 
 #include <errno.h>
 #include <string.h>
@@ -13,7 +14,11 @@
 #include "utils.h"
 
 
-const char *regnames[] { "Control", "Status", "Error register 1", "Maintenance", "Attention summary", "Desired sector/track address", "Look ahead", "Drive type", "Serial no", "Offset", "Desired cylinder address", "Current cylinder address", "Error register 2", "Error register 3", "ECC position", "ECC pattern" };
+constexpr const int NSECT       = 22;               // sectors per track
+constexpr const int NTRAC       = 19;               // tracks per cylinder
+constexpr const int SECTOR_SIZE = 512;
+
+constexpr const char *regnames[] { "Control", "Status", "Error register 1", "Maintenance", "Attention summary", "Desired sector/track address", "Look ahead", "Drive type", "Serial no", "Offset", "Desired cylinder address", "Current cylinder address", "Error register 2", "Error register 3", "ECC position", "ECC pattern" };
 
 rp06::rp06(bus *const b, std::atomic_bool *const disk_read_activity, std::atomic_bool *const disk_write_activity) :
 	b(b),
@@ -76,9 +81,14 @@ uint16_t rp06::read_word(const uint16_t addr)
 	return value;
 }
 
+int rp06::reg_num(uint16_t addr) const
+{
+	return (addr - RP06_BASE) / 2;
+}
+
 void rp06::write_byte(const uint16_t addr, const uint8_t v)
 {
-	uint16_t vtemp = registers[(addr - RP06_BASE) / 2];
+	uint16_t vtemp = registers[reg_num(addr)];
 
 	if (addr & 1) {
 		vtemp &= ~0xff00;
@@ -92,19 +102,61 @@ void rp06::write_byte(const uint16_t addr, const uint8_t v)
 	write_word(addr, vtemp);
 }
 
+uint32_t rp06::compute_offset() const
+{
+        // cyl num, track num, sector num, which were written like this:
+        uint16_t cn = registers[reg_num(RP06_DC)];
+        uint16_t tn = (registers[reg_num(RP06_DA)] >> 8) & 0377;
+        uint16_t sn = registers[reg_num(RP06_DA)] & 0377;
+
+        // each cylinder is NSECT*NTRAC sectors
+        // each track is NSECT sectors
+        uint32_t offs = cn * NSECT * NTRAC;
+        offs += tn * NSECT;
+        offs += sn;
+        offs *= SECTOR_SIZE;
+
+        return offs;
+}
+
+uint32_t rp06::getphysaddr() const
+{
+	constexpr const uint16_t A16 = 0400;
+	constexpr const uint16_t A17 = 01000;
+
+	// low 16 bits in UBA, and tack on A16/A17
+	bool cur_A16 = registers[reg_num(RP06_CS1)] & A16;
+	bool cur_A17 = registers[reg_num(RP06_CS1)] & A17;
+
+	uint16_t cur_A1621 = 0;
+
+	// but also bits may be found in bae... the assumption here is
+	// if these bits are non-zero they override A16/A17 but they
+	// really need to be consistent...
+	if (registers[reg_num(RP06_BAE)]) {
+		cur_A16 = false;  // subsumed in A1621
+		cur_A17 = false;  // subsumed
+		cur_A1621 = registers[reg_num(RP06_BAE)] & 077;
+	}
+
+	return registers[reg_num(RP06_UBA)] | (cur_A16 << 16) | (cur_A17 << 17) | (cur_A1621 << 16);
+}
+
 void rp06::write_word(const uint16_t addr, uint16_t v)
 {
-	const int reg = (addr - RP06_BASE) / 2;
+	const int reg = reg_num(addr);
 
 	TRACE("RP06: write \"%s\"/%06o: %06o", regnames[reg], addr, v);
 
         registers[reg] = v;
 
-	if (reg == RP06_CR) {
+	if (reg == RP06_CS1) {
 		if (v & 1) {
-			int function_code = (v >> 1) & 31;
+			int function_code = v & 63;
 
+			if (function_code == 070) {
 
+			}
 		}
 	}
 }
diff --git a/rp06.h b/rp06.h
index 6c6015c..724e1ae 100644
--- a/rp06.h
+++ b/rp06.h
@@ -1,5 +1,6 @@
 // (C) 2024 by Folkert van Heusden
 // Released under MIT license
+// Some of the code is translated from Neil Webber's PDP11/70 emulator
 
 #pragma once
 
@@ -15,9 +16,20 @@
 #include "disk_backend.h"
 
 
-#define RP06_CR 0176700  // control register
-#define RP06_BASE  RP06_CSR
-#define RP06_END  (RP06_MPR + 2)
+#define RP06_CS1 	0176700  // control register
+#define RP06_WC	 	0176702  // word count
+#define RP06_UBA	0176704  // UNIBUS address
+#define RP06_DA	 	0176706  // desired address
+#define RP06_CS2	0176710  // control/status register 2
+#define RP06_DS	 	0176712  // drive status
+#define RP06_AS	 	0176716  // unified attention status
+#define RP06_RMLA	0176720  // lookahead (sector under head!!)
+#define RP06_OFR	0176732  // heads offset
+#define RP06_DC	 	0176734  // desired cylinder
+#define RP06_CC	 	0176736  // "current cylinder" and/or holding register
+#define RP06_BAE	0176750  // address extension (pdp11/70 extra phys bits)
+#define RP06_BASE  RP06_CS1
+#define RP06_END  (RP06_BAE + 2)
 
 
 class bus;
@@ -27,11 +39,15 @@ class rp06: public disk_device
 private:
 	bus      *const b;
 
-	uint16_t registers[20] { };
+	uint16_t registers[32] { };
 
 	std::atomic_bool *const disk_read_activity  { nullptr };
 	std::atomic_bool *const disk_write_activity { nullptr };
 
+	int      reg_num(uint16_t addr) const;
+	uint32_t getphysaddr() const;
+	uint32_t compute_offset() const;
+
 public:
 	rp06(bus *const b, std::atomic_bool *const disk_read_activity, std::atomic_bool *const disk_write_activity);
 	virtual ~rp06();